In recent years, a so-called tandem type image forming apparatus including a plurality (four) of image forming stations has been proposed. Such an image forming apparatus is capable of quickly forming a color image using an electrophotographic process, and therefore, receives attention.
In each image forming station, around a photosensitive member, a charging device (charging means), an exposure device (exposure means) and developing device (developing means) are provided. Further, toner images formed at the respective image forming stations are successively transferred superposedly onto an intermediary transfer member (image receiving member), and thereafter, are transferred onto a recording material altogether.
Here, as a type of the charging device, two types, i.e., an “AC charging type” for (electrically) charging the photosensitive member by applying a superposed voltage of an AC voltage and a DC voltage and a “DC charging type” for (electrically) charging the photosensitive member by applying only the DC voltage have been known.
The “AC charging type” is advantageous in that a surface of the photosensitive member can be uniformly charged compared with the “DC charging type”, but a discharge amount to the photosensitive member is large and therefore the photosensitive member tends to be liable to deteriorate. Further, an expensive AC voltage (power) source is needed. On the other hand, compared with the “AC charging type”, in the “DC charging type”, the photosensitive member does not readily deteriorate, but tends to be inferior in charging uniformity. That is, compared with the “DC charging type”, the “AC charging type” is high in initial costs and running costs. In other words, compared with the “AC charging type”, the “DC charging type” is advantageous in terms of the running costs and the initial costs.
Therefore, in the case where the “DC charging type” is intended to be employed, the following problems can generate.
Specifically, in an apparatus described in Japanese Laid-Open Patent Application 2002-189400, in order to lower a potential (residual potential) of the photosensitive member remaining after transfer to the neighborhood of 0 V, a device for optically discharging the photosensitive member, a so-called pre-exposure device (discharging means), is mounted. Thus, in the case where a method of discharging the photosensitive member by using the pre-exposure device is employed, in the charging device, the photosensitive member has to be charged from the neighborhood of 0 V to a desired potential (e.g., −700 V), and compared with the case where a discharging step by the pre-exposure device is not performed, a discharge current becomes large. That is, compared with the case where the discharging step by the pre-exposure device is not performed, the photosensitive member tends to be liable to deteriorate.
On the other hand, in the case where the discharging step is not performed without providing the pre-exposure device (there is an advantage of cost reduction), the photosensitive member only has a discharging effect by a transfer device. This discharging effects depends on a toner image formed in a preceding image forming station, i.e., an amount of a toner which is a resistor, and in the case where this amount is large, the step goes to a subsequent charging step while the photosensitive member little has the discharging effect. Particularly, when each of toner images formed in first and second image forming stations has a maximum density (two color solid images), in transfer steps of third and fourth image forming stations, the photosensitive members little have the discharging effect. Resulting from this, there is a liability that a ghost image (defective image) generates on a subsequent image. That is, in the third and fourth stations, from their arrangement viewpoint, a density of a coming toner image tends to become high, so that there is a liability that the ghost image can generate.
On the other hand, in the first image forming station, from its arrangement viewpoint, there is no coming toner image, and therefore there is no liability that the ghost image generates.